Stefan Kahil Lessard is the eldest child of Janaki and Ron Lessard. Both of his parents were musicians that played frequently at weddings and lounges. After moving a number of times during his childhood, Lessard and his family eventually settled in Charlottesville, Virginia in 1987, where he enrolled in the Tandem Friends School, where he studied music under trumpeter John D'earth. John D'earth suggested Stefan to Dave Matthews, who was looking for a bass player, when he was forming the Dave Matthews Band. Stefan joined the band when he was only 16 years old.
Stefan dropped out of his junior year of high school to focus more on his music career. He was accepted at Richmond's Virginia Commonwealth University in their jazz studies program, but left after only six weeks, when it became he realized that he couldn't do his school work while continuing with the band.
Lessard married long-time girlfriend Josie Baucom in 1997. They had two children. One son together, Elijah Diego (born January 9, 1998) and lost a daughter, Aslan, to SIDS in 1996. They divorced in 2004.
In October 2005 he married Jaclyn Burton, and together they have two daughters, Hazel Bailey (born March 6, 2004) and Flora Jude (born July 13, 2006).
On November 29, 2007, Lessard's home in western Albemarle County, Virginia, burned down. The Lessard family had a total loss estimated at over 2 million dollars. It took over 40 firefighters from five different fire departments to extinguish the blaze. The origin of the fire is unknown, but the Albemarle County Fire Marshall's Office cites the structure's cedar shake siding as a contributor to the rapid spread of the fire. Lessard and his son were home at the time of the fire, though they escaped unharmed.